1. Field of the Invention
The invention relates to sensors for performing electrochemical analysis to determine concentrations of analytes in mixtures of liquids.
2. Discussion of the Art
Electrochemical assays for determining the concentrations of enzymes or their substrates in complex mixtures of liquids have been developed. In particular, biosensor strips for biomedical applications (e.g., whole blood analyses) have been developed for the detection of glucose levels in biological samples. In general, the biosensor strips comprise typical electrochemical cells in which there can be working electrodes, counter electrodes, and pseudo reference/counter electrodes. The potential of the working electrode is typically kept at a constant value relative to that of the pseudo reference/counter electrode.
Biosensor strips are used in the chemical industry, for example, to analyze complex mixtures. They are also used in the food industry and in the biochemical engineering industry. Biosensor strips are also useful in medical research or in external testing. In medical research, they can function as invasive probes (i.e., where they come into contact with a body fluid, such as whole blood or subcutaneous fluid). In external testing, they can function in a non-invasive manner (i.e., where they come into contact with blood withdrawn by a syringe or a pricking device).
A typical three-electrode sensor for blood analysis suitable for measuring the amount of analyte in a sample of liquid comprises (1) an active or working electrode that is coated with a layer containing an enzyme and a redox mediator, (2) a passive or dummy electrode that is coated with a layer containing a redox mediator but lacking an enzyme, and (3) a pseudo reference/counter electrode or counter electrode. When a sample of liquid containing a species for which the enzyme is catalytically active contacts the electrodes, the redox mediator transfers electrons in the catalyzed reaction. When a voltage is applied across the electrodes, a response current results from the reduction or oxidation of the redox mediator at the electrodes. The response current at the dummy electrode represents a background response of the electrode in contact with the sample. The response current at the working electrode is related to the concentration of the substrate. A corrected response current is calculated by subtracting the response current at the dummy electrode from the response current at the working electrode. This subtraction calculation substantially eliminates background interferences, thereby improving the signal-to-noise ratio in the sensor.
Non-monotonic current decay can occur in a system when the resistance between the working electrode and the pseudo reference/counter electrode is large. This type of current decay can complicate measurements of concentration of analyte.
In general, this invention provides an electrochemical cell having a working electrode having an auxiliary area that contains a redox species. The auxiliary area provides a current path of low resistance between the working electrode and a pseudo reference/counter electrode. The auxiliary area is an integral part of the working electrode. The auxiliary area allows an enhanced current to flow. The enhanced current helps to reduce or even substantially eliminate non-monotonic decay of the current transient. The auxiliary area of the working electrode is generally located closer to the pseudo reference/counter electrode than is the working area of the working electrode.
The auxiliary area of the working electrode does not contribute to an electrochemical measurement of the analyte of interest, because there is no catalytic component (e.g., enzyme) in the auxiliary area. As a result, the current associated with the auxiliary area is generated from simple oxidation, or reduction, of the redox species and follows a Cottrellian response. In the electrode configuration of this invention, the additive effect of the current is significant only during the first few seconds of the response, correcting any non-monotonic behavior of the current decay.
The enhanced current has no net effect during the period of time during which the measurement is being made, because the duration of the period is short. In a system containing two electrodes (e.g., a working electrode and a dummy electrode), an auxiliary area can be applied to the dummy electrode. The auxiliary area of the dummy electrode must be of the same size and shape as that of the auxiliary area of the working electrode. The auxiliary area of the dummy electrode and the auxiliary area of the working electrode are preferably positioned symmetrically with respect to the pseudo reference/counter electrode. This configuration produces an identical response at both auxiliary areas. Therefore, any effect on the measurement current is canceled out upon subtraction of the current of one electrode from the current of the other electrode.
In one aspect, the invention provides an electrochemical cell comprising a first electrode, which is referred to as a working electrode. The first electrode comprises a first working area and a first auxiliary area. The first working area comprises a working ink. The first auxiliary area comprises a first dummy ink. The working ink comprises an enzyme and a first redox mediator. The first dummy ink comprises a redox species, but lacks an enzyme. The redox species of the first dummy ink can be the same material as the redox mediator in the working ink.
The electrochemical cell can include a second electrode, which is referred to as a dummy electrode. The second electrode comprises a second working area and a second auxiliary area. The second working area comprises a second dummy ink, which comprises the first redox mediator, but lacks an enzyme. The second auxiliary area comprises the first dummy ink.
The electrochemical cell can include a pseudo reference/counter electrode. The first auxiliary area is preferably located closer to the pseudo reference/counter electrode than is the first working area. The second auxiliary area is preferably located closer to the pseudo reference/counter electrode than is the second working area.
The first working area can be larger in area than the first auxiliary area. The second working area can be larger in area than the second auxiliary area. The first working area can be smaller in area than the second working area and second auxiliary area in combination.
In another aspect, the invention provides a biosensor strip. The strip comprises an electrode support, a first electrode, i.e., a working electrode, a second electrode, i.e., a dummy electrode, and a pseudo reference/counter electrode. Each of the electrodes is disposed on and supported by the electrode support. The pseudo reference/counter electrode is spaced apart from the first electrode and second electrode. The biosensor strip can include a covering layer, which defines an enclosed is space over the electrodes. The covering layer has an aperture for receiving a sample for introduction into the enclosed space. The biosensor strip can also include at least one layer of mesh interposed in the enclosed space between the covering layer and the electrodes.
In another aspect, the invention provides a method of manufacturing an electrochemical cell. The method includes the steps of applying a working ink to a first electrode to form a first working area, and applying a first dummy ink to the first electrode to form a first auxiliary area. The first electrode is a working electrode. The method can also include the steps of applying a second dummy ink to a second electrode to form a second working area, and applying the first dummy ink to the second electrode to form a second auxiliary area. The second electrode is a dummy electrode.
Under some conditions, the current decay at the working electrode departs from the expected model. In particular, it is expected that the current will decrease monotonically over time and exhibit the behavior predicted by the Cottrell equation. However, when the dummy electrode imposes a significant current load on the pseudo reference/counter electrode, the current at the working electrode departs from classical behavior and may actually increase over some short time period. Glucose meters with which the biosensor strips of this invention are used have electronic features designed to detect invalid test results. One of these electronic features involves monitoring the current decay at the working electrode. If this decay is not monotonic, the meter will report an error condition and abort the test.
The auxiliary areas can reduce or substantially eliminate non-monotonic current decay during the first few seconds of a chronoamperometric test. Accordingly, it is possible to reduce or even eliminate the occurrence of errors on electrochemical sensors used to analyze blood glucose. The auxiliary areas on the dummy electrode and on the working electrode can help overcome errors by increasing the initial current spike of the working electrode and the dummy electrode. The current increase results from oxidation or reduction of the additional redox species introduced by the dummy ink. The current generated from the oxidation or reduction of the redox species has a low resistance path to the pseudo reference/counter electrode, which allows efficient oxidation or reduction of the redox species without substantial voltage drops.
Each of the first redox mediator and the redox species can be a ferrocene. Preferably, the enzyme is glucose oxidase.
Other features and advantages of the invention will be apparent from the descriptions of the embodiments thereof.